This invention relates, in general, to electronic components and, more particularly, to chemical sensing devices and methods of making the same.
Chemical sensing devices are used in applications where it is necessary to detect the presence of a gas in an ambient and indicate the presence of the gas to an integrated circuit. For example, a carbon monoxide detector comprises a sensor that enables an integrated circuit to sound an alarm when the sensor detects the presence of carbon monoxide.
Such chemical sensors are typically formed by micro-machining a silicon substrate and forming a sensing element on the silicon substrate. The sensing element consists of a layer of chemical sensing material, such as tin oxide, that is formed on a heating element. The heating element is used to heat the layer chemical sensing material to approximately 350 degrees centigrade (.degree.C.). At the elevated temperature, the resistivity of the layer of chemical sensing material changes due to the presence of the gas to be detected. For example, the resistivity of tin oxide changes significantly when heated to 350.degree. C. and placed in the presence of carbon monoxide. The change in resistivity can be measured and be used to indicate the presence of carbon monoxide.
Thus, the layer of chemical sensing material must be heated to a high temperature and that temperature must be maintained during operation. Due to the conductive nature of the materials commonly used to form chemical sensors (e.g., silicon substrates), there is a significant loss of thermal energy as the chemical sensor is operated. To reduce this thermal loss, and thus the amount of energy that is consumed by a chemical sensor, the heating element and chemical sensing material are formed on a thin membrane that is connected to a silicon substrate. The process used to form the thin membrane includes an anisotropic etch to remove the bulk of the silicon substrate under the heating element.
The process of forming the thin membrane introduces several problems when the chemical sensors are formed in a high volume production operation. First, it is difficult to control the etch rate of the anisotropic etch so the final thickness and uniformity of the thin membrane will vary from one manufacturing lot to the next and even across a wafer in a manufacturing lot. If the membrane is thinned too much, the anisotropic etch can damage the heating element and the layer of chemical sensing material. Another problem associated with this process is that it is necessary to align the anisotropic etch process, which occurs on the backside of the silicon substrate, to the process used to form the heating element and chemical sensing material on the front side of the silicon substrate. Backside alignment techniques not only complicate the manufacturing process, but add significant cost to the chemical sensor.
By now it should be appreciated that it would be advantageous to provide a method of making a chemical sensor that did not require the formation of a thin membrane to provide thermal isolation. It would also be advantageous if the method did not require the use of backside alignment techniques as part of the manufacturing process used to form the chemical sensor.